A rotary dental file is an instrument used to perform a reaming operation during the course of an endodontic procedure. The goal of the procedure is to remove dead or damaged material from the root canal of a tooth prior to filling the excavated canal. Examples of instruments of this kind are referred to in U.S. Pat. No. 4,934,934, entitled “Dental File/Reamer Instrument, issue on Jun. 19, 1990, to Arpaio, Jr. et al. and U.S. Pat. No. 5,017,138, entitled “Set of Endodontic Instruments”, issued on May 21, 1991 to Schilder.
In order to use such an instrument, the shank of the rotary file is inserted into the chuck of a dental drilling instrument, or into a handle that may be directly manipulated by a dentist. In operation, an endodontic drill is operated at a speed of approximately three hundred revolutions per minute. This rotary speed achieves the necessary surface velocity for the various protrusions or edges projecting from the file that are adapted to cut, abrade, shape and excavate the root canal of the tooth undergoing the endodontic treatment. The relatively low rotation speed is accompanied by a correspondingly high drive torque that is transferred to the instrument as it rotates along with the chuck.
The surface and the shape of the root canal are irregular. As the surface of the rotary file enters the root canal and encounters the relatively hard canal wall surface, the high driving torque generated by the drill creates the risk that the working part of the rotary file will break off, fracture, or otherwise separate into two pieces. This is especially true for endodontic files made of nickel titanium. File separation typically leaves some part of the rotary file embedded in the root canal. The fracture usually occurs about two millimeters from the file tip. Consequently the embedded, separated file tip is quite difficult to extricate from the tooth. Sometimes the tip can only be removed by a specialist, and in some cases the tip is left in the patient's tooth. Rotary file breakage may occur suddenly and without warning, even with a new file. Breakage can occur regardless of the brand of file, its design, age, length, taper or size.
Past attempts to address the problem of rotary file torsional failure have incorporated the concept of manufacturing an intermediate part of the file shank so as to have a torsional fracture strength that is substantially less than that of the remainder of the file shank. An example of such a device is disclosed in U.S. Pat. No. 5,586,885, entitled “Dental File/Reamer Instrument”, issued on Dec. 24, 1996 to Kert. The goal of the Kert arrangement is to place any fracture of the rotary file outside of the root canal, making it possible to remove the embedded portion of the file by gripping the protruding portion. However, the Kert device has not succeeded in clinical practice because the characteristics of the annular groove used to weaken the shank are not disclosed. In particular, the dimensions and shape of the groove are not discussed in sufficient detail to permit precise use on files of different diameters, lengths and tapers. The result is a rotary file that either fractures so readily as to be useless or that does not fracture when needed, resulting in a failure in another portion of the file near the tip prior to the failure of the weakened area.
The result is that rotary file manufacturers have addressed the file breakage problem by focusing on improved endodontic techniques and by encouraging the use of new files for each endodontic procedure. These efforts have not solved this widely recognized and frequently occurring problem.